The present disclosure relates to a modular rotor craft rotor hub system and methods of assembling the same, and more particularly, to a modular rotor craft rotor hub system that includes fully articulated rotor arm assemblies having discrete bearings for each degree of freedom.
A key component of a rotor craft is the main rotor hub system. It provides attachment of the main rotor blades during operation. Rotational power is delivered to the main rotor hub system to provide rotational velocity to the blades in order to create aerodynamic lift. The main rotor hub system must allow for rotational motion of the blades in the vertical (flap), horizontal (lead-lag), and axial (pitch) directions near the blade root attachment with the hub to accommodate flight control authority and dynamic stability. Main rotor hub systems that accommodate these motions with discrete hinge mechanisms are referred to as fully articulated hub systems.
At least some known fully articulated rotor hub systems provide beneficial design kinematics, but struggle to provide these rotational freedoms with bearing systems that can accommodate high frequency and high amplitude oscillatory motion under high thrust loading created by the centrifugal force of the rotating blades. One known hub system is a non-friction bearing system such as a ball or roller bearing system. The lubricants and seals of these types of bearing systems are susceptible to moisture extrusion and leakage and therefore demand frequent maintenance that often requires removal and disassembly of the entire rotor hub to service. Another known hub system is a strap pack hub system that includes stretch straps formed from expensive specialized steel. At least some known strap pack hubs systems experience severe and complicated loadings and therefore stress states, resulting in strict damage criteria and frequent replacement often requiring removal and disassembly of the hub. Furthermore, failure of non-friction and strap pack hub systems may be difficult to detect and their low damage tolerance may quickly lead to aircraft damage or failure. Moreover, many known non-friction and strap pack hub systems currently perform at a maximum power limit and may not be able to handle an increase in induced loads, within their current physical envelops, without failure.
It has been known for some time that the use of elastomeric bearings in a rotor hub system would eliminate weight, the need of lubrication and would minimize maintenance. As such, at least some known rotor hub systems include spherical elastomeric bearings to accommodate for the flap and pitch degrees of freedom such that these are not handled by discrete bearings but by a single spherical bearing. As a result of consolidating these motions, the dynamic qualities of the rotor hub system have to be carefully considered, modeled, and controlled to assure aircraft stability. For this reason, replacing many legacy hubs, such as non-friction and strap pack hub systems, with an elastomeric hub system utilizing spherical bearings for flap compliance would entail a large design and analysis effort, often being cost prohibitive. More specifically, spherical elastomeric bearings have similar footprints to non-friction and strap pack hubs, but the dynamics and kinematics are much different, requiring significant research and development costs to implement a spherical elastomeric bearing hub system on an aircraft having strap-pack or roller bearing based legacy hubs.